Access forbidden when editing/deleting a block from the block layout with a machine name beginning with "." dot

This came up as the daily BSI target

On 11.x I can confirm this is still an issue

In the block.schema.yml I see

      type: machine_name
      label: 'ID'
      # Blocks have atypical machine names: they allow periods for historical reasons.
      # @see https://www.drupal.org/project/drupal/issues/2685917
      # @see https://www.drupal.org/project/drupal/issues/2043527
      constraints:
        Regex:
          pattern: '/^[a-z0-9_.]+$/'
          message: "The %value machine name is not valid."

And the issue this was committed on 📌 Add config validation for the allowed characters of machine names Fixed

I think the regex and schema should be updated so it can't start or end with a period.

added regex to schema and blockForm and updated the existing test coverage in providerInvalidMachineNameCharacters()

Think the summary needs to be updated and cleared up